(a) Field of the Invention
The present invention relates to a semiconductor device having a recessed-channel-array MOSFET, and a method for manufacturing the same.
(b) Description of the Related Art
DRAM (Dynamic Random Access Memory) devices include an array of memory cells for storing therein information. Each memory cell includes a MOSFET formed in the surface region of a silicon substrate and a cell capacitor connected to the MOSFET, and stores electric charge in the cell capacitor by controlling the MOSFET. In recent years, the line width of interconnections in the DRAM device is drastically reduced along with the attempt of a higher memory capacity in the DRAM device. The reduction in the line width also reduces the distance between the source and the drain of the MOSFET, thereby necessitating use of a countermeasure for preventing a short-channel effect in the MOSFET.
As one of the countermeasures for preventing the short-channel effect, a recessed-channel-array MOSFET (RCAT) is known in the art. In the RCAT, a doped-polysilicon film configuring a layer of the gate electrode of the RCAT is embedded in a recess formed in the surface region of the silicon substrate. The structure of the RCAT and a method for manufacturing the same are described in a literature entitled “2003 Symposium on VLSI Technology Digest of Technical Papers”, p. 11-12, for example.
In the RCAT, the recessed channel formed along the surface of the recess having a U-characteristic shape enlarges the effective channel length, thereby preventing the short-channel effect of the MOSFET.
However, there is a problem in the RCAT that the gate electrode of the RCAT has a larger parasitic capacitance compared to the conventional MOSFET because the gate electrode opposes the recessed channel having the larger length. The larger parasitic capacitance generally causes a lower operational speed of the MOSFET. For reducing the parasitic capacitance of the gate electrode in the RCAT, it may be considered to increase the thickness of the associated gate oxide film. However, the increase of the thickness of the gate oxide film increases the threshold voltage of the RCAT, thereby reducing the operational speed and increasing the power dissipation of the RCAT.